The invention relates generally to photonic crystals, and relates more particularly to resonant coupling in photonic crystal circuits.
FIG. 1 is a perspective view illustrating a conventional photonic crystal device 100, e.g., configured for use as a resonant cavity filter. Specifically, the photonic crystal device 100 comprises a substrate 102, a periodic lattice comprising a plurality of apertures 104 formed through the substrate, an optical cavity 106 formed within the periodic lattice (e.g., by omitting a series of apertures), and a photonic crystal waveguide 108 also formed within the periodic lattice and side-coupled to the optical cavity 106.
In operation, light 110 comprising a plurality of signals (e.g., f1, f2, . . . , fn) at different wavelengths enters the photonic crystal waveguide 108, which functions as a waveguide bus. As the light 110 approaches the side-coupled optical cavity 106, which typically has very strong wavelength selectivity, signals 112 of a certain wavelength or wavelengths (e.g., fi) will be filtered or coupled to the optical cavity 106, while the remainder of the light 110 continues to propagate through the waveguide bus 108.
A problem with such a configuration is that while the waveguide bus should ideally have wavelength-independent properties for optimal filtering performance, photonic crystal waveguides such as those used for the waveguide bus actually tend to have very strong wavelength selectivity, both in amplitude transmission and in phase sensitivity. Specifically, large group velocity dispersion, which can distort optical pulses propagating along the waveguide bus, and very small bandwidth for low-loss propagation are inherent properties of photonic crystal waveguides. Thus, they are not ideal for use as waveguide buses, e.g., as used in resonant cavity filtering.
Thus, there is a need for a method and an apparatus for resonant coupling in photonic crystal circuits.